We propose to study the role of Fc receptors in antigen-induced B lymphocyte actavition in young and aged mice. B lymphocytes will be immunized with DNP-Ficoll in a Mishell-Dutton tissue culture system. Under these conditions, addition of 400 micrograms of mouse aggregated or non-aggregated immunoglobulin at the beginning of culture inhibits 40% of the plaque forming cell response occurring by 96 hours of culture. We plan to investigate the following points: 1) Determine the binding to B lymphocytes of agg and non agg IgGl, IgGla and IgG2b of Fc and (Fab1)2 fragments; 2) Determine the inhibitory effect of the best binder and of its Fc and (Fab1)2 fragments; 3) Determine the inhibitory effect of antigen-antibody complexes; 4) Determine the inhibitory dose response for each Fc receptor ligand on DPFC and IPFC; 5) Determine mode of optimal depression; 6) Isolate specific DNP binding cells and study DNP-F binding SMIg and the movement of this complex with and without Fc receptor ligands. These studies should help to clarify the role of Fc receptors in antigen-induced B cell activation in young and aged mice and possible mechanisms of age-induced immunodepression.